Question

How many atoms of hydrogen are there in 18 g of water?

Answer 1

The number of moles of water is calculated by dividing the mass of water by the molecular weight of water. The one mole of any substance is equal to the $6.023 \times {10^{23}}$ units of atoms, molecules or ions.

Complete step by step answer:
The mass of water is 18 g.
The molecular weight is calculated by adding the atomic weight of the atoms and multiplying the number of atoms with the atomic number.
The atomic weight of hydrogen is 1.
The atomic weight of oxygen is 16.
The molecular weight is calculated as shown below.
$\Rightarrow Molecular\;weight = 1.00794 \times 2 + 15.9994$
$\Rightarrow Molecular\;weight = 18.015$
The molecular weight of water is 18.015 g/mol.
As the mass of the water and molecular weight is known, the number of moles of water can be calculated.
The formula to calculate the number of moles is shown below.
$n = \dfrac{m}{M}$
Where,
n is the number of moles
m is the mass
M is the molecular weight
To calculate the moles of water, substitute the value in the above formula.
$\Rightarrow n = \dfrac{{18g}}{{18.015g/mol}}$
$\Rightarrow n = 0.999mol \approx 1mol$
The chemical formula of water is ${H_2}O$, in water two mole of hydrogen is present and one mole of oxygen is present.
So, in a single molecule of water, two mole of hydrogen are present.
One mole of water contains $6.023 \times {10^{23}}$ number of water molecules or
$\Rightarrow 2 \times 6.023 \times {10^{23}} = 12.046 \times {10^{23}}$ atoms of hydrogen
18 g of water will contain $\Rightarrow 1 \times 12.046 \times {10^{23}} = 12.046 \times {10^{23}}$ atoms of hydrogen.
Therefore, $12.046 \times {10^{23}}$ atoms of hydrogen are present in 18 gram of water.

Note:
The value $6.023 \times {10^{23}}$ is known as Avagadro’s number or Avagadro’s constant. Don’t get confused when 2 mole is multiplied with the Avogadro's number as the value 2 denotes the number of hydrogen atoms present in one molecule of water.